I want to download WhatsApp from App Store

Start here: Update your iPhone, iPad, or iPod touch - Apple Support

Then here: Get apps in the App Store on iPhone - Apple Support

If you’re on an iPhone 6 or earlier, your options for apps and downloads is limited, as older iPhone models are no longer supported, cannot be upgraded to current iOS, and no hardware upgrades are available. Which leaves replacement with a newer (used, refurb) or new iPhone, for apps requiring recent or current iOS versions.